In the heating and cooling of relatively large buildings, a number of problems are encountered. Building codes usually require a predetermined minimum air flow to meet ventilation requirements resulting in the supplying of conditioned air to zones independent of their thermostatic requirements. One result can be overcooling which, typically, exists at the start of a work day. Since thermostatic response would be wrong for heating and the heating would not be fast enough at the minimum flows required for ventilation purposes, a temporary switchover of both the air supply and thermostatic response is necessary. As the various terminals open and/or throttle, the plenum static pressure changes which must also be accounted for in the operation of the controller in order to maintain a stable operation.